1. Field of the Invention
This invention relates to moving picture encoder and, more particularly, to a moving picture encoder which is suitably used for systems for digital transmitting and storing of moving pictures with reduced bit rate, particularly television conferences, television telephones, digital television transmission, digital video storage systems, etc.
2. Description of the Prior Art
In the prior art, moving picture encoding has been made by utilizing both spatial correlation and temporal correlation.
The moving picture encoding utilizing the temporal correlation is carried out on the basis of the fact that two adjacent picture frames have many similar portions (that is, have correlation). Thus, in this method, only portions which are not similar are quantized in conversion areas. An example of encoder which realizes this method will now be described with reference to the block diagram of FIG. 1.
Referring to FIG. 1, a subfactor 71 obtains the difference between an input picture X1 and a predicted picture X2 of 8.times.8 pels, for instance, to generate a prediction error picture X3. Designated at numeral 72 is a converter for performing discrete cosine transform (DCT) or the like, at numeral 73 a quantizer, at numeral 74 an inverse quantizer for inversely quantizing quantized signal X4, and at numeral 75 an inverse converter for performing inverse discrete cosine transform (IDCT) or the like. An adder 76 adds together prediction error picture X5 restored by the inverse converter 75 and the predicted picture X2 to generate local decode picture X6. A predictor 77 generates the predicted picture X2 according to the local decode picture X6.
With this encoder which utilizes the temporal correlation for moving picture encoding, it is possible to greatly compress information. However, the picture signal of moving picture has local temporal correlation fluctuations. For example, in a scene where an object is moving in a certain background, the background itself and the object are similar in moving pictures. However, when a portion which has been concealed by the object in the preceding picture is revealed with the movement of the object, no similar scene to that in the preceding picture can not be found. At this time, it is preferred to directly encode the present picture signal rather than taking the difference between the prediction signal obtained from the preceding picture and the present picture signal. In other words, depending small area on picture the prediction utilizing the temporal correlation is or is not suitably carried out to obtain better encoding efficiency.
Heretofore, the encoding algorithm has been switched for each small area of picture. Of two different modes, i.e., one in which the small areas are directly encoded (intra mode), and one in which the prediction error signal is encoded (inter mode), either mode involving generation of less information is selected for encoding the small areas.
FIG. 2 is a block diagram showing an example of inter/intra mode switch encoder in the case of using the DCT. Referring to FIG. 2, designated at 78 is the inter/intra mode switch. The other reference numerals designate parts like those in FIG. 1.
The inter/intra mode switch 78 is operative for each block of 8.times.8 pels to select the mode which involves generation of less information at the time of quantization. With the switch 78 thrown to terminal 78a, the intra mode (i.e., direct encoding mode) is selected, and thrown to terminal 78b the inter mode (i.e., difference encoding mode) is selected.
When the intra mode and inter mode are compared for each conversion coefficient or each sub-band, the encoding mode which is suited for the encoding efficiency improvement may be different in dependence on the conversion coefficient or sub-band even in the same small area of 8.times.8 pels, for instance. As is seen from FIG. 2, for instance, in the inter mode the quantization error is also fed back. That is, not only the necessary signal component but also error signal is encoded and transmitted. This is inefficient particularly at the time of high compression subject to high noise power. However, the inter mode is still effective because low frequency components have high power compared to the error signal.
However, where the whole small area is dealt with either in the inter mode or in the intra mode as in the prior art, there occur conversion coefficients or sub-bands for which the encoding is done in the inadequate mode in view of the encoding efficiency improvement, thus resulting in encoding efficiency reduction.